This invention relates to an improved method of producing soft porcelain characterized by whiteness and elegant translucency by using a minimum amount of bone phosphate, which is a by-product of the process of producing gelatin from ox bones.
Conventionally, when bone china, which is a representative soft porcelain, is produced, bone ash (calcined ox bone), namely 3Ca.sub.3 (PO.sub.4).Ca(OH).sub.2, is used as a main raw material. However, the production of bone ash involves many difficulties in the selection of the raw material and in the firing method.
Thus, a method capable of obtaining easily and cheaply a raw material, which is more stable in quality than bone ash, has been investigated. It has been found that bone phosphate which can be obtained in the course of producing gelatin from cattle bones by treating it with hydrogen chloride and separating the gelatin component and neutralizing the remaining waste liquid with milk of lime, can be used instead of bone ash since the impurities contained in the thus obtained bone phosphate are small in quantity. There are several methods of producing soft porcelain by use of the thus obtained bone phosphate. One of the most advanced methods is disclosed in Japanese Pat. No. 679,226. The method of preparing a green body in this patent is characterized by calcining the by-product bone phosphate at temperatures of 1,000.degree. to 1,250.degree. C. to convert it to calcined bone phosphate, which is slightly soluble in water, followed by mixing (a) 28 to 40 wt% of the calcined bone phosphate, (b) 6 to 14 wt% of limestone, (c) 20 to 35 wt% of kaolin (part of which can be replaced by "Gairome-clay"), (d) not more than 15 wt% of china stone, (e) 5 to 30 wt% of feldspar, and (f) 0 to 10 wt% of quartz.
The conventional calcined bone phosphate, is a mixture of calcium phosphate component and a comparatively large quantity of .beta.-calcium pyrophosphate, which is obtained by calcining bone phosphate at temperatures of 1,000.degree. to 1,250.degree. C. The calcined bone phosphate, mixed in a green body, is apt to produce foam in the body while it is fired since the calcium phosphate component other than .beta.-calcium pyrophosphate, undergoes changes during the firing process.